1.  Not sure what day registration opens next year, but it was April 1, this year.  The race sells out anywhere from two days to two weeks.  This year, I think it was like 10 days.  Your best bet (and only sure thing) is to register the day it opens.  You'll know when it is from the website plenty in advance.

2.  Is there really a "good" marathon for under-five-year-olds   D.C. is awesome and tons to do/see.  I always bring my family for this one, but my kids are older and run the 10K with their dad and are old enough to see and enjoy all the museums and history and government.  And they enjoy watching and participating in the marathon/10K.  Not sure what options are open in the area for entertaining little ones.  Maybe someone else with young kids can shed some light.

3.  I've ran very hill, rolling hills, and flat marathons and never have seemed to have thought of MCM as being really hilly.  There's a couple of spots, but overall, it is an easy course, though a little crowded for my taste, but lots of crowd support.  And of course there's that last 200 yards straight uphill to the finish, but hey, it's the FINISH!  Who cares at that point.  Nice young virile handsome Marines waiting to put a medal around your neck... sorry, got sidetracked...

You'll love this one.  I'm from Texas and it's flat here too pretty much (except Austin).  You won't have any problem.  Just get registered ASAP.

1.  Anyone who wants to get into the MCM can.  The only folks who can't get in are the folks who procrastinate.  Just visit the MCM main web page in Feb 2010 and they will announce the registration date.    If you register within a few days of opening, you will be ok.

2.  All of the Smithsonian museums in DC are free.  Your children can go to the Air and Space to see all of the planes or the Natural History Museum and see some pre-historic dinosaurs.  The national zoo and the national aquarium are also in the city.

3.  I would rank the MCM as one of the more flat races course that I've run.  They have a gradual climbl at the beginning but it's not bad since you are full of energy and there are so many people at the begining that you won't even notice the uptrend.   It's over before you know it.  You'll get rewarded with a nice downhill run into Georgetown afterwards.  The other hills include a ramp up the 14th Street Bridge and Iwo Jima at the end.  the Iwo Jima hill is like 50 yards.  Everything else in between is pretty much flat as a pancake.

Exatly.  And the earlier you book, the cheaper.  Last year I paid $300/night at the Residence Inn Rosslyn.  I reserved it again for THIS year right after last year's MCM and got the same room for $200/night.  Get a hotel EARLY, way before registration.